Crude oil is generally recovered from an oil-bearing reservoir by three processes, designated primary, secondary and tertiary recovery. The latter is also known as Enhanced Oil Recovery. In primary recovery the oil is produced through a producer well by taking advantage of the pressure exerted on underground pools of oil by gas or water present with the oil. Approximately 20% of the original oil in place (OOIP) is recovered by this process. Once the pressure has been exhausted, other means of recovering the remaining oil must be employed. In secondary recovery the well may be re-pressurized with gas or water injected through one or more injection wells to recover approximately an additional 20% of the OOIP. Other secondary recovery methods include acidizing, fracturing, water flood, etc. After secondary recovery means have been exhausted, EOR processes can be employed to recover additional oil up to approximately 60% OOIP.
Many EOR techniques have been disclosed in the past yet the EOR process is not widely used by the industry for several reasons. For example, in the EOR processes employing chemicals, petroleum sulfonates and synthetic alkylaryl sulfonates are predominantly used as the surfactant to lower the interfacial tension (IFT) between the residual oil and the injection fluid in order to overcome the capillary forces trapping the oil. Partially hydrolyzed polyacrylamides are generally employed as the viscosifier for mobility control. Both the polymers and surfactants used are not salt and multivalent cation tolerant and therefore either a fresh water source or pre-treatment of the injection water is required. Also, a costly hydration unit is often required for the polymer in order to properly dissolve and develop its viscosity. Furthermore, often a high concentration of the surfactant is required for proper oil displacement, or, alkali is used with the surfactant to enhance the interfacial tension and reduce the surfactant adsorption. In addition, the polyacrylamide may precipitate and cause serious formation damage when contacting the connate water containing multivalent cations. Most polymers are not stable at temperatures above 140° C. and are irreversibly degraded by shear. The huge up-front investment and product limitations currently discourage the wide use of the EOR process. This is especially the case for many marginal fields, owned by smaller independent companies even though the fields contain considerable residual oil reserves.
The present invention relates to the composition and process for injecting amphoteric viscoelastic surfactant gels into a subterranean oil bearing formation through one or more injection wells, displacing the fluid into the formation and recovering oil from one or more production wells. The viscoelastic surfactant gels contain amphoteric surfactants, commonly known as betaines. These are used alone or in combination with certain other surfactants and/or polymers in an aqueous injection fluid, to reduce the interfacial tension (IFT) between brines and crude oils and increase the viscosity of the injection brine.
There are many citations in the prior art that have recognized the use of various amphoteric surfactants for water flood and other oil recovery processes. U.S. Pat. No. 3,939,911 describes a three-surfactant system containing a water soluble salt of an alkyl or alkylaryl sulfonate, a phosphate ester surfactant and sulfonated betaine for oil recovery in formations having high temperature and high concentrations of polyvalent ions. U.S. Pat. No. 4,130,491 describes the use of naphthenic acid based betaines for recovering mineral oil from oil deposits.
U.S. Pat. No. 4,193,452 employs a surface active amphoteric quaternary ammonium sulfonate and an aliphatic alcohol containing 5 to 8 carbon atoms to produce a thicken injection fluid for waterflooding. U.S. Pat. No. 4,216,097 uses amphoteric surfactants having an inner quaternary ammonium group linked to a terminal sulfonated or carboxylate group to reduce the oil-water interfacial tension in relatively high salinity aqueous media that include the presence of significant quantities of divalent metal ions. The amphoteric betaines used in this case are based on tertiary amines they do not increase the viscosity of the injected fluid as do the alkyl amido betaines of the present invention. They are primarily used to lower the IFT between the injection fluid and the residual oil. As explained in the example included in U.S. Pat. No. 4,216,097, xanthan gum was used as a mobility control agent.
U.S. Pat. No. 4,554,974 discloses a method for recovering petroleum from a subterranean reservoir by injecting an aqueous slug of a mixture of amphoteric surfactant to reduce the interfacial tension and a high molecular weight polysaccharide gum as the thickener.
U.S. Pat. No. 4,825,950 uses a betaine as an IFT lowering surfactant along with two different polymers to form an electrolyte tolerant injection fluid for oil recovery. The first polymer is primarily for thickening and the second is used to prevent the first polymer and the surfactant from interfering with each other through interaction and/or precipitation.
U.S. Pat. No. 6,831,108 and recent US Patent Applications 2004/0082484 and 2004/0176478 use zwitterionic/amphoteric surfactants such as dihydroxyl alkyl glycinate, alkyl amido ampho acetate or propionate, alkyl betaine, alkyl amidopropyl betaine and alkyl imino mono and di-proprionates derived from certain waxes, fats and oils in viscoelastic surfactant based aqueous fluids systems useful as thickening agents in conjunction with an inorganic water-soluble salt or organic additives such as phthalic acid, salicylic acid or their salts for suspending particles during the excavation of geologic formations. Also the intent of the viscoelastic fluids of the above citations is not to recover residual oil from subterranean reservoirs by injecting through one or more injection wells and producing the oil from one or more production wells, as is the intent of the present invention.
The recent US Patent Application 2004/0214725 employs quaternary ammonium salts and alkyl amido amine salts of inorganic acids and/or organic acids to produce viscoelastic aqueous systems. These fluids are suggested for use in fracturing, gravel packing, drilling, completion fluids, as well as several non-oilfield based applications
None of these examples of the prior art disclose the use of amphoteric surfactants, preferably alkyl amido betaines, specifically chosen and designed to be used in aqueous injection fluids and especially in produced brine without any water treatment or water softening to provide both low IFT and mobility control for the recovery of residual oil by injection into one or more injection wells, displacing the fluid into the formation, and recovering the oil from one or more production wells.